[aylons]

Well, you may not have to worry, but if you have large unpoured areas on a design with a professional PCB manufacturer (of the traditional, high-touch kind), they will ask if you want to pour some copper there. Reason being that it makes the process faster, more consistent and reduce possible side-etching on lanes. It may not a make a difference in most cases, but you may just save some time and effort by doing this.

[iancmceachern]

The reason is that the copper is already there, it gets etched away. So it actually costs more to not have copper than to have it.

[shermantanktop]

At industrial scale, surely they recover that copper from the etching fluid?

[aylons]

Yes, but this costs pales in the cost of redoing in case of problems with side-etching and the overall tightened manufacturing constraints.

And yes, they get to recover the copper, at the very least to make treatment easier for discarding (copper is a very bad pollutant). But not only there's a cost, this is dealt with by waste treatment companies that will at most use the copper value to recoup some of the cost of the treatment.